(1) Field of the Invention
This invention relates to a rack guide in a rack-and-pinion type steering gear for an automobile and the like, and more particularly to a rack guide designed to prevent an abnormal noise created between rack teeth and a pinion.
(2) Description of the Prior Art
As shown in FIGS. 3 and 4, a rack-and-pinion type steering gear includes a rack bar 1 arranged in a lateral direction of a vehicle, a pinion 2 provided at an end of a steering shaft (not shown) and meshed with the rack bar 1, and link rods 3 mounted at both ends of the rack bar 1. The line rods 3 are adapted to move the direction of right and left knuckle arms (not shown) of both wheels thereby to effect a steering operation. The rack bar 1 is supported by a rack guide 4 on a side thereof opposite to a meshed side of the pinion 2, while the rack guide 4 is prepressurized by a spring 5 and an adjustment screw 6 on an opposite side of the rack bar 1. Conventionally, the rack guide 4 is made mostly of synthetic resin. The following problems have not been solved.
Generally, synthetic resin moldings are annealed to allow uniform crystallization after demolding, and they tend to be shrunk during annealing. Such shrinkage occurs differently in dependence upon a product shape and a wall thickness distribution, etc. In the case of the rack guide 4 as mentioned above, the rack guide 4 is shrunk to form a frusto-conical shape. As a result, a clearance C is created between an inner circumferential surface 7a of a guide hole of a rack housing 7 and an outer circumferential surface 4a of the rack guide 4. As a result, during a rough road running of the vehicle, when an external force is randomly applied to the rack bar 1, the rack bar 1 receives a repulsive force from the pinion 2 based on a pressure angle and a lead angle of rack teeth 1a, and accordingly receives a motive force in an axial direction (as depicted by an arrow A) of the pinion 2. Such a motive force is also transmitted to the rack guide 4. As a result, the movement of the rack bar 1 in the axial direction of the pinion 2 may not be restricted by the frusto-conical rack guide 4 because of the presence of the clearance C as shown in FIG. 4, and there is created an abnormal metallic noise between the rack teeth 1 a and the pinion 2. To reduce such an abnormal noise, the rack guide 4 is conventionally machined after annealing to form a columnar shape thereof which may be fitted to the inner circumferential surface 7a of the guide hole. Alternatively, the rack guide is molded in a die whose inner face is designed to make a columnar molding after the shrinkage due to annealing. However, die face matching of such a die is difficult and increases the cost of the die.